GLOW PHENOMENON AND CRYSTALLIZATION - EVIDENCE THAT THEY ARE DIFFERENT EVENTS FOR HAFNIUM ZIRCONIUM MIXED OXIDES

If hydrous zirconia is heated at a controlled rate, an exothermic event occurs at about 723 K. Several investigators attributed this to a “glow phenomenon,” while others termed this a “crystallization” process. These two events occur at the same temperature (723 K) for zirconia. However, evidence is presented to show that crystallization and the glow phenomenon occur at different temperatures for hafnia and Hf/Zr mixed oxides. In the first step of the transformation, the amorphous material undergoes a crystallization process to yield small crystals which diffract X-rays and possess a high surface area. In the second transformation, these small crystals coalesce to form larger particles which contain multiple domains since the crystal size obtained from X-ray line broadening does not change significantly during the exothermic event. The present results demonstrate that crystallization and the exothermic events, commonly referred to as a glow phenomenon, may be two separate events. © 1993 Academic Press, Inc.